Gordon Brown is already ruffling feathers after being nicknamed 'the real Deputy Prime Minister' by Sir Keir Starmer's inner circle.
While Justice Secretary David Lammy officially has the title, Mr Brown has ousted him from the plum Downing Street office he was given to go with the position.
Mr Brown's appointment to an unpaid, part-time advisory role last weekend was part of Sir Keir's last-ditch attempt to save his job – and the 75-year-old New Labour titan is looking to assert himself now he is back, insiders say.
The 'real Deputy PM' moniker is likely to sting Mr Lammy, who is believed to enjoy the kudos of his ceremonial title.
Mr Lammy is even said to demand his ministry officials call him 'Deputy Prime Minister' – even though it is not his main job – with some complaining they are wasting time on his 'vanity post'.
Sources close to Mr Lammy deny those claims and insist his new office in the Dover House HQ of the Scotland Office is 'bigger and better' than Downing Street.
They say it is far more suitable for a Deputy PM and 'somewhere you can host international visitors.'
But the nickname has only heightened tensions among Sir Keir's top team at a fraught time.
The former Prime Minister's appointment last week as Special Reviewer on Global Finance and Cooperation was met with anger among some Labour MPs.
Prime Minister Keir Starmer appoints Gordon Brown as Special Envoy on Global Finance and Cooperation at 10 Downing Street
Many in the parliamentary party were furious that sitting MPs were overlooked by Sir Keir in favour of Mr Brown.
Mr Brown will report directly to Sir Keir and is tasked with building international finance partnerships that can support defence and security in the UK.
He will prepare the ground before the UK assumes the presidency of the G20 group next year.
A spokesman for Mr Lammy dismissed the claims and said: 'The Deputy Prime Minister was given a new office in Dover House back in April.'